'Das Ich ist nicht aus Leib und Seele zusammengesetzt, sondern es ist eine bestimmte Entwicklungsstufe des Wesens, das von verschiedenem Standpunkt betrachtet in körperliches und geistiges Dasein auseinanderfällt.'—Wundt, Vorlesungen über die Menschen-und Thierseele, i. 293.
INTRODUCTION.
In no respect has the progress of physical science exercised a more profound influence upon philosophical thought than it has by proving an apparently quantitative relation between material changes and mental changes. It has always been known that there is qualitative relation. Even long before mankind suspected that the brain was in any way connected with thought, it was well understood that alcohol and other poisons exercised their sundry influences on the mind in virtue of influences which they exercised upon the body; and even the lowest savages must always have been aware that a blow on the head is followed by insensibility. But it was not until the rise of Physiology that this qualitative relation between corporeal changes and mental changes was gradually found to be a quantitative one—or that every particular change of mind had an exact and invariable counterpart in some particular change of body. It is needless for me to detail the successive steps in the long course of physiological discovery whereby this great fact has been established; it is enough to say that the fact is established to the satisfaction of every physiologist.
Now, when once the relation between material changes and mental changes has been thus recognized as quantitative—or, which is the same thing, when once the association has been recognized as both invariable and exact—there arises the question as to how this relation is to be explained. Formally considered—or considered as a matter of logical statement irrespective of the relative probabilities which they may present, either to the minds of different individuals or to the general intelligence of the race—it appears to me that the possible hypotheses are here seven in number.
I. The mental changes may cause the material changes.
II. The material changes may cause the mental changes.
III. There may be no causation either way, because the association may be only a phenomenal association—the two apparently diverse classes of phenomena being really one and the same.